The only load i had was a 12V 50W automotive lamp, as the actual load to
be used is about 3,000 miles away.

The requirement of that beastie was minimum dropout, so i looked at a
number of 12V "LDO" regulators, and they either had 1V drop at almost
all current loads, or the drop was 400-700mV.
The only decent LDO regulator i found was the LM2940CT-12, so i
decided to parallel them using a series resistor of 0.2 ohms at the
output of each one toward the load.
I used a 47uF cap right at the regulator output in conformance to the
datasheet reccomendation.
To reduce dissipation of the regulators at full load and 15V max
input, i used four 2.7 ohm 5W resistors.
For testing, i used a FWB battery charger output (bypassed its
regulator) and a Variac to adhust the peak output voltage.
The only load i had was a 12V 50W automotive lamp, as the actual load
to be used is about 3,000 miles away.
It looks good, and the dropout at low inputs (ie: slightly above 12V
output) is excellent.
BUT....
...here is the problem.
I see spike-type oscillation at the input, apparently when the input
voltage is high enough for those 4 "shunt" or "bypass" resistors to pass
current.
There aer zero spikes when this regulator board is not used: ie: when
the battery charger is driving the load (lamp) directly.

Q: What is causing this spiking, and how can it be fixed?

The requirement of that beastie was minimum dropout, so i looked at a
number of 12V "LDO" regulators, and they either had 1V drop at almost
all current loads, or the drop was 400-700mV.
The only decent LDO regulator i found was the LM2940CT-12, so i
decided to parallel them using a series resistor of 0.2 ohms at the
output of each one toward the load.
I used a 47uF cap right at the regulator output in conformance to the
datasheet reccomendation.
To reduce dissipation of the regulators at full load and 15V max
input, i used four 2.7 ohm 5W resistors.
For testing, i used a FWB battery charger output (bypassed its
regulator) and a Variac to adhust the peak output voltage.
The only load i had was a 12V 50W automotive lamp, as the actual load
to be used is about 3,000 miles away.
It looks good, and the dropout at low inputs (ie: slightly above 12V
output) is excellent.
BUT....
...here is the problem.
I see spike-type oscillation at the input, apparently when the input
voltage is high enough for those 4 "shunt" or "bypass" resistors to pass
current.
There aer zero spikes when this regulator board is not used: ie: when
the battery charger is driving the load (lamp) directly.

Q: What is causing this spiking, and how can it be fixed?

MooseFET wrote:
On Apr 13, 9:10 pm, Robert Baer <robertb...@localnet.com> wrote:

The requirement of that beastie was minimum dropout, so i looked at a
number of 12V "LDO" regulators, and they either had 1V drop at almost
all current loads, or the drop was 400-700mV.
The only decent LDO regulator i found was the LM2940CT-12, so i
decided to parallel them using a series resistor of 0.2 ohms at the
output of each one toward the load.
I used a 47uF cap right at the regulator output in conformance to the
datasheet reccomendation.
To reduce dissipation of the regulators at full load and 15V max
input, i used four 2.7 ohm 5W resistors.
For testing, i used a FWB battery charger output (bypassed its
regulator) and a Variac to adhust the peak output voltage.
The only load i had was a 12V 50W automotive lamp, as the actual load
to be used is about 3,000 miles away.
It looks good, and the dropout at low inputs (ie: slightly above 12V
output) is excellent.
BUT....
...here is the problem.
I see spike-type oscillation at the input, apparently when the input
voltage is high enough for those 4 "shunt" or "bypass" resistors to pass
current.
There aer zero spikes when this regulator board is not used: ie: when
the battery charger is driving the load (lamp) directly.

Q: What is causing this spiking, and how can it be fixed?

Did you put in the capacitors the LDO needs right near it? Are your
resistors inductive?

The caps are as close as the parts allow; the power resistors are
wier wound but the spikes are in the tens to hundreds of microseconds
wide with similar but larger seperation.
Spike rate increases as input voltage increases.

"Robert Baer"


The only load i had was a 12V 50W automotive lamp, as the actual load to
be used is about 3,000 miles away.



** That is an awfully long cable run to drive a 12 volt load.

Wouldn't it be easier to move the supply closer ?



..... Phil



....ain't easy to move the mountain or the prophet...

On Apr 13, 9:10 pm, Robert Baer <robertb...@localnet.com> wrote:

The requirement of that beastie was minimum dropout, so i looked at a
number of 12V "LDO" regulators, and they either had 1V drop at almost
all current loads, or the drop was 400-700mV.
The only decent LDO regulator i found was the LM2940CT-12, so i
decided to parallel them using a series resistor of 0.2 ohms at the
output of each one toward the load.
I used a 47uF cap right at the regulator output in conformance to the
datasheet reccomendation.
To reduce dissipation of the regulators at full load and 15V max
input, i used four 2.7 ohm 5W resistors.
For testing, i used a FWB battery charger output (bypassed its
regulator) and a Variac to adhust the peak output voltage.
The only load i had was a 12V 50W automotive lamp, as the actual load
to be used is about 3,000 miles away.
It looks good, and the dropout at low inputs (ie: slightly above 12V
output) is excellent.
BUT....
...here is the problem.
I see spike-type oscillation at the input, apparently when the input
voltage is high enough for those 4 "shunt" or "bypass" resistors to pass
current.
There aer zero spikes when this regulator board is not used: ie: when
the battery charger is driving the load (lamp) directly.

Q: What is causing this spiking, and how can it be fixed?



Did you put in the capacitors the LDO needs right near it? Are your
resistors inductive?
The caps are as close as the parts allow; the power resistors are

On Sun, 13 Apr 2008 21:10:27 -0700, Robert Baer
robertbaer@localnet.com> wrote:


The requirement of that beastie was minimum dropout, so i looked at a
number of 12V "LDO" regulators, and they either had 1V drop at almost
all current loads, or the drop was 400-700mV.
The only decent LDO regulator i found was the LM2940CT-12, so i
decided to parallel them using a series resistor of 0.2 ohms at the
output of each one toward the load.
I used a 47uF cap right at the regulator output in conformance to the
datasheet reccomendation.
To reduce dissipation of the regulators at full load and 15V max
input, i used four 2.7 ohm 5W resistors.
For testing, i used a FWB battery charger output (bypassed its
regulator) and a Variac to adhust the peak output voltage.
The only load i had was a 12V 50W automotive lamp, as the actual load
to be used is about 3,000 miles away.
It looks good, and the dropout at low inputs (ie: slightly above 12V
output) is excellent.
BUT....
...here is the problem.
I see spike-type oscillation at the input, apparently when the input
voltage is high enough for those 4 "shunt" or "bypass" resistors to pass
current.
There aer zero spikes when this regulator board is not used: ie: when
the battery charger is driving the load (lamp) directly.

Q: What is causing this spiking, and how can it be fixed?


You haven't indicated whether the non-uniform current drain has an
effect on output regulation. If not, it could just be that you are
seeing the 100mA variation of quiescent current that occurs as a
single unit sees minimum headroom. You might consider ignoring it, if
no functional problems result, until proper test equipment and loads
allow a proper solution to become practical.
** The variable load (due to thermal changes) of the lamp appears to not

As output voltages are not identical (and even when they are), there
is likely to be motor-boating around the threshold.
** Gets worse as input voltage increses.

Placing series output resistors on an LDO seems rather ham-fisted.
You've not mentioned your specified performance aims; it's not really
possible to offer constructive alternatives until you do.
** Just followed the same scheme recommended for the 1, 2, and 5A LDOs

Obviously 220mV of load regulation is not considered to be a problem -
this could qualify a lot of LDOs with nominally higher headroom
requirements, as their spec will have assumed a much tighter
regulation requirement in the spec formulation.

The LD29300 goes up to 3A @400mV, which could reduce your component
count and potential modes of irregular parallel performance.
** If i have too much trouble with the presen design, i may consider

A discrete circuit is an obvious choice, if you're already going to
the trouble of hooking up 5 linears, but I assume you're stuck out in
the wilderness with only a soldering iron and a Digikey catalog
delivery service.

RL
** Good call!

On Apr 15, 1:46 am, Robert Baer <robertb...@localnet.com> wrote:

MooseFET wrote:

On Apr 13, 9:10 pm, Robert Baer <robertb...@localnet.com> wrote:

The requirement of that beastie was minimum dropout, so i looked at a
number of 12V "LDO" regulators, and they either had 1V drop at almost
all current loads, or the drop was 400-700mV.
The only decent LDO regulator i found was the LM2940CT-12, so i
decided to parallel them using a series resistor of 0.2 ohms at the
output of each one toward the load.
I used a 47uF cap right at the regulator output in conformance to the
datasheet reccomendation.
To reduce dissipation of the regulators at full load and 15V max
input, i used four 2.7 ohm 5W resistors.
For testing, i used a FWB battery charger output (bypassed its
regulator) and a Variac to adhust the peak output voltage.
The only load i had was a 12V 50W automotive lamp, as the actual load
to be used is about 3,000 miles away.
It looks good, and the dropout at low inputs (ie: slightly above 12V
output) is excellent.
BUT....
...here is the problem.
I see spike-type oscillation at the input, apparently when the input
voltage is high enough for those 4 "shunt" or "bypass" resistors to pass
current.
There aer zero spikes when this regulator board is not used: ie: when
the battery charger is driving the load (lamp) directly.

Q: What is causing this spiking, and how can it be fixed?

Did you put in the capacitors the LDO needs right near it? Are your
resistors inductive?

The caps are as close as the parts allow; the power resistors are
wier wound but the spikes are in the tens to hundreds of microseconds
wide with similar but larger seperation.
Spike rate increases as input voltage increases.


What was the part number of this LDO so I can remember never to use
it?

It still sounds like it is oscillating. Some LDOs oscillate if the
ouput side capacitor is too good. You may need to put a resistance in
series with the output side capacitor.
NOTES:

MooseFET wrote:

On Apr 15, 1:46 am, Robert Baer <robertb...@localnet.com> wrote:

MooseFET wrote:

On Apr 13, 9:10 pm, Robert Baer <robertb...@localnet.com> wrote:


The requirement of that beastie was minimum dropout, so i looked at a
number of 12V "LDO" regulators, and they either had 1V drop at almost
all current loads, or the drop was 400-700mV.
The only decent LDO regulator i found was the LM2940CT-12, so i
decided to parallel them using a series resistor of 0.2 ohms at the
output of each one toward the load.
I used a 47uF cap right at the regulator output in conformance to the
datasheet reccomendation.
To reduce dissipation of the regulators at full load and 15V max
input, i used four 2.7 ohm 5W resistors.
For testing, i used a FWB battery charger output (bypassed its
regulator) and a Variac to adhust the peak output voltage.
The only load i had was a 12V 50W automotive lamp, as the actual load
to be used is about 3,000 miles away.
It looks good, and the dropout at low inputs (ie: slightly above 12V
output) is excellent.
BUT....
...here is the problem.
I see spike-type oscillation at the input, apparently when the input
voltage is high enough for those 4 "shunt" or "bypass" resistors to
pass
current.
There aer zero spikes when this regulator board is not used: ie: when
the battery charger is driving the load (lamp) directly.


Q: What is causing this spiking, and how can it be fixed?


Did you put in the capacitors the LDO needs right near it? Are your
resistors inductive?


The caps are as close as the parts allow; the power resistors are
wier wound but the spikes are in the tens to hundreds of microseconds
wide with similar but larger seperation.
Spike rate increases as input voltage increases.



What was the part number of this LDO so I can remember never to use
it?

It still sounds like it is oscillating. Some LDOs oscillate if the
ouput side capacitor is too good. You may need to put a resistance in
series with the output side capacitor.

NOTES:
The LT1083CP-12 data sheet, page 9 shows how to parellel them and it
seems that this scheme of output resistors is good for the 1A to 7.5A
units.
In looking at datasheeets for the MIC29500, the LM2937ET-12 and the
LF1200V type parts, that there *IS* mention of caps with too low ESR but
one of the datasheets has a graph that shows stability from 0.001 up to
some max - so there is no indication as to exactly (or
evenapproximately) what the acceptable low end ESR might be: 20
micro-ohms???
There is also mention of instability if there is a thermal overload,
but a 25 ohm resistive load shows one spike as the input goes above the
threshold.
And there is a statement that one could ignore oscillations (say what?).
I am beginning to think that the "shunt"/"bypass" resistors have
something to do with the problem - maybe a crazy idea of feed-forward
gain (there is a lag from output change to the LDO output due to the
series resistor and the 47uF capacitor).
I am going to try removing some of those 2.7 ohm resistors to see if
ther is any waveform difference.
**
Worst case, i will have to toss what i have and try the MIC29752WWT.
BTW,how does one find the ESR of a capacitor when the only spec is
100KHz impedance?
I do not want to buy a bunch of differnt caps and then attempt to
measure ESR without a decent meter (or metering circuit to build).
Spiking is exactly the same using four 2.7 ohm resistors in parallel

NOTES:
The LT1083CP-12 data sheet, page 9 shows how to parellel them and it
seems that this scheme of output resistors is good for the 1A to 7.5A
units.
In looking at datasheeets for the MIC29500, the LM2937ET-12 and the
LF1200V type parts, that there *IS* mention of caps with too low ESR but
one of the datasheets has a graph that shows stability from 0.001 up to
some max - so there is no indication as to exactly (or
evenapproximately) what the acceptable low end ESR might be: 20
micro-ohms???
There is also mention of instability if there is a thermal overload,
but a 25 ohm resistive load shows one spike as the input goes above the
threshold.
And there is a statement that one could ignore oscillations (say what?).
I am beginning to think that the "shunt"/"bypass" resistors have
something to do with the problem - maybe a crazy idea of feed-forward
gain (there is a lag from output change to the LDO output due to the
series resistor and the 47uF capacitor).
I am going to try removing some of those 2.7 ohm resistors to see if
ther is any waveform difference.
**
Worst case, i will have to toss what i have and try the MIC29752WWT.
BTW,how does one find the ESR of a capacitor when the only spec is
100KHz impedance?
I do not want to buy a bunch of differnt caps and then attempt to
measure ESR without a decent meter (or metering circuit to build).
Spiking is exactly the same using four 2.7 ohm resistors in parallel
for the "shunt"/"bypass" as using two in parallel.
With zero resistors (ie: "normal" datasheet configuration), there is
definite very large oscillations: step up and semi-RC ramp down on both
input and output. This is obviously the case of thermal shutdown as
mentioned in the datasheet - unacceptable.
with only one 2.7 ohm resistor, the oscillations are faster with
pulses more narrow. this might actually be better.

On Tue, 15 Apr 2008 21:21:42 -0700, Robert Baer
robertbaer@localnet.com> wrote:



NOTES:
The LT1083CP-12 data sheet, page 9 shows how to parellel them and it
seems that this scheme of output resistors is good for the 1A to 7.5A
units.
In looking at datasheeets for the MIC29500, the LM2937ET-12 and the
LF1200V type parts, that there *IS* mention of caps with too low ESR but
one of the datasheets has a graph that shows stability from 0.001 up to
some max - so there is no indication as to exactly (or
evenapproximately) what the acceptable low end ESR might be: 20
micro-ohms???
There is also mention of instability if there is a thermal overload,
but a 25 ohm resistive load shows one spike as the input goes above the
threshold.
And there is a statement that one could ignore oscillations (say what?).
I am beginning to think that the "shunt"/"bypass" resistors have
something to do with the problem - maybe a crazy idea of feed-forward
gain (there is a lag from output change to the LDO output due to the
series resistor and the 47uF capacitor).
I am going to try removing some of those 2.7 ohm resistors to see if
ther is any waveform difference.
**
Worst case, i will have to toss what i have and try the MIC29752WWT.
BTW,how does one find the ESR of a capacitor when the only spec is
100KHz impedance?
I do not want to buy a bunch of differnt caps and then attempt to
measure ESR without a decent meter (or metering circuit to build).

Spiking is exactly the same using four 2.7 ohm resistors in parallel
for the "shunt"/"bypass" as using two in parallel.
With zero resistors (ie: "normal" datasheet configuration), there is
definite very large oscillations: step up and semi-RC ramp down on both
input and output. This is obviously the case of thermal shutdown as
mentioned in the datasheet - unacceptable.
with only one 2.7 ohm resistor, the oscillations are faster with
pulses more narrow. this might actually be better.


The paralleling method for LT1083 adjustable devices is not applicable
to a fixed regulator, particularly one from another mfr with a
differing construction.

2.7 ohm resistors? What...where...how.....why....
You were using ~200mOhm parts in series with the output of 1A
regulators (LM2940) to assist in sharing.

The use of tantalum output caps above 22uF/per or low ESR AL
electrolytics (at room temp) 100uF/per should suffice for ESR targeted
by the LM2940 device. Note that the concern re esr is only really
applicable below 200mA/per. The 220mOhm/per swamping resistor should
cover low end limitations for ESR, under all conditions, so load the
caps on to the common load point with a clear conscience.

I'm not sure what you are describing as 'spikes'. Is this a voltage or
current irregularity (if current, how measured)? Rep rate, amplitude
and duration, in each instance, please.

RL
The LT1083 is a fixed voltage regulator, which makes your statement

The paralleling method for LT1083 adjustable devices is not applicable
to a fixed regulator, particularly one from another mfr with a
differing construction.

2.7 ohm resistors? What...where...how.....why....
You were using ~200mOhm parts in series with the output of 1A
regulators (LM2940) to assist in sharing.

The use of tantalum output caps above 22uF/per or low ESR AL
electrolytics (at room temp) 100uF/per should suffice for ESR targeted
by the LM2940 device. Note that the concern re esr is only really
applicable below 200mA/per. The 220mOhm/per swamping resistor should
cover low end limitations for ESR, under all conditions, so load the
caps on to the common load point with a clear conscience.

I'm not sure what you are describing as 'spikes'. Is this a voltage or
current irregularity (if current, how measured)? Rep rate, amplitude
and duration, in each instance, please.

RL
The LT1083 is a fixed voltage regulator, which makes your statement
slightly confusing.

The 2.7 ohm "shunt"/"bypass" resistors are connected from the input
to the load and are to share current when the input goes above the
regulated voltage - mainly to decrease regulator dissipation.

The 0.2 ohm resistors in series with the regulators are to balance
the load amoung them.

The (square-wave looking) spikes at the input are roughly 200uSec
wide and spaced roughly 400uSec; depends on the input drive.
Amplitude is 16V at peak and about 12.4V at bottom.

The LT1083 is a fixed voltage regulator, which makes your statement
slightly confusing.

On Wed, 16 Apr 2008 21:16:30 -0700, Robert Baer
robertbaer@localnet.com> wrote:





The LT1083 is a fixed voltage regulator, which makes your statement
slightly confusing.


There may be some valid reasons for confusion here.

There are two data sheets, one describing the LT1083 as fixed, the
other as being adjustable.

fixed
http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1003,C1040,C1055,P1278,D2603

adjustable
http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1003,C1040,C1055,P1281,D3741

Fixed versions are 'dashed' voltage ie LT1083-5 LT1083-12

Pax?

RL
Check; i think i may have mentioned the LT1083-12; presently i am